This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus which is regulated to produce a charged surface area on a photoconductive member corresponding in size substantially to that of a sheet of support material arranged to have a copy of an original document reproduced thereon.
A typical electrophotographic printing machine includes a photosensitive surface which is electrostatically charged substantially uniformly thereover. A light image of the original document being reproduced irradiates the charged photosensitive surface. This light image discharges the electrostatic charge in the irradiated areas of the photosensitive surface. As a consequence thereof, the remaining charge on the photosensitive surface forms an electrostatic charge pattern in image configuration, i.e. an electrostatic latent image. The electrostatic latent image is then developed by contacting it with finely divided electrostatically attractable material, i.e. toner particles. The toner particles adhere electrostatically to the photosensitive surface in a pattern corresponding to that of the latent image recorded thereon. Thereafter, the developed image is transferred to the sheet of support material, e.g. paper or any other suitable material, such as a thermoplastic sheet. The powder image transferred to the sheet of support material is subsequently permanently affixed thereto forming a permanent print of the original document thereon. This process is more thoroughly described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is similar to the heretofore discussed process. However, each image in the multi-color electrophotographic printing process represents a partial single color image which corresponds to one color of the original document. Thus, a multi-color process requires the utilization of a plurality of single color images each adapted to be reproduced with its correspondingly complementarily colored toner particles. Accordingly, a color reproduction requires a plurality of differently colored toner powder images which are superimposed onto the sheet of support material in registration with one another, thereby forming a color copy. The formation of the color copy from a colored original document requires substantially more toner particles than is employed in the creation of black and white copies. This substantially increases the risk of contaminating various machine components with toner particles.
A uniform charge potential is applied to the photoconductive surface by a corona generating device. Various types of corona generating devices may be used in electrophotographic printing. For example, U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958 and U.S. Pat. No. 2,778,946 issued to Mayo in 1957 disclose suitable corona generating devices for use in electrophotographic printing. However, regardless of the type of corona generating apparatus employed, all of the heretofore utilized devices are energized continuously and charge the entire surface of the photoconductive member. This results in the non-image areas being developed with toner particles. These toner particles must be subsequently cleaned from the photosensitive surface and introduce the additional risk of contamination.
In order to alleviate this problem some machines employ inter-image erase lamps. An inter-image erase lamp discharges the photoconductive surface in the non-image regions This prevents the subsequent development of the non-image regions and reduces the waste and contamination resulting from the deposition of toner particles in these areas. The employment of an inter-image erase lamp introduces an added complexity to the electrophotographic printing machine. Inter-image erase lamps generally comprise an electro-luminescent panel which must be periodically excited by the machine logic to discharge only the selected region of the photoconductive surface, i.e. the non-image region.
Other prior art devices have been employed wherein a photoconductive copy sheet is employed to actuate a high intensity lamp and a corona generating unit. In this type of an apparatus the leading edge of the copy sheet actuates the high intensity lamp and corona generating unit. Thereafter, the trailing edge of the copy sheet de-energizes the foregoing assemblies. One example of this type of apparatus is disclosed in U.S. Pat. No. 3,700,327 issued to Murgas in 1972. Other patents which disclose various techniques for controlling the corona generating apparatus are U.S. Pat. No. 2,890,343 issued to Bolton in 1959, U.S. Pat. No. 3,335,273 issued to Walkup in 1967 U.S. Pat. No. 3,791,732 issued to Mihalik et al in 1974, and U.S. Pat. No. 3,444,354 issued to Mindell in 1969. However, none of the foregoing prior art patents appear to discuss the problem associated with continuously charging the photoconductive member.
Accordingly, it is a primary object of the present invention to improve the corona generating apparatus so that the size of the area charged on the photoconductive surface conforms substantially to that of the support sheet having the copy of the original document being reproduced thereon.